Few things can startle like a lawn mower that starts with a loud bang. It not only shocks you, but also shocks your unsuspecting neighbours.
That happens when your mower backfires, meaning the fuel has combusted outside the engine.
Why does a lawn mower backfire when trying to start it? Common causes include:
- Bad fuel
- Overheating
- Dirty air filter
- Dirty spark plug
- Faulty carburetor
- Poor carburetor adjustment
- Faulty flywheel key
- Ignition delay
- Compression problems
- Faulty valves
If your mower has backfired, it’s important to identify the cause so you can prevent it from happening again.
That keeps your machine in good condition and gives you (and your neighbourhood) peace of mind.
Backfiring vs. Afterfiring
Both backfire and afterfire cause loud noises (such as a pop or bang) from a small engine.
The noise comes from fuel coming into contact with air and heat outside of the engine – such as in the exhaust – causing combustion. This usually results in a loud bang and possibly a small burst of fire.
Backfire happens with the engine running – whether while starting an engine, running it, or shutting it down. For example, backfire can happen when your mower starts then dies.
Afterfire happens when the engine is already shut off. It can be caused by shutting the engine off at a too-high RPM, muffler design, or a malfunctioning engine solenoid.
Why Does a Lawn Mower Backfire When Starting?
It can be nerve-racking to start your mower only to hear a sudden bang and feel the motor stop. Don’t panic – and move away so you’re not injured.
When checking what causes the engine to backfire when starting, start with fuel, air, and spark. If these aren’t an issue, take a look at other components.
Before inspecting your lawn mower, shut it off and let it cool down completely. Disconnect the spark plug wire to prevent accidental starts.
Bad fuel
This is one of the more common causes of a small engine backfire.
“Bad gas” can come from:
- Stale or old gas (e.g. left in the engine for a long time)
- Poor-quality gas (e.g. too much ethanol)
- Contaminated gas (e.g. mixed with water)
Moisture can get into the oil or fuel if your lawn mower gets wet, so make sure it’s properly covered when in storage.
SOLUTION: You’ll have to inspect the fuel tank and source. Check the blend – if there’s a high amount of ethanol or a high mix of alcohol, it’s not suitable for a small engine.
On the other hand, stale or contaminated gas may not combust properly.
The best fix is to drain the tank and fill it up with good-quality gas. Always follow proper gas recommendations for a small engine.
Overheating
Your mower can overheat during use, but it can also overheat before use. If the weather is too hot, it can warm up the engine and even cause parts to misalign.
The heat then causes the gas to ignite prematurely when starting, or it ignites any fuel pooled in the exhaust – causing a backfire.
SOLUTION: Nothing to do but wait for cooler weather! Besides, if it’s too hot for your mower, it’s too hot for you as well.
The best time to mow is mid-morning, after any dew has evaporated but before the sunlight is too harsh. Additionally, store your mower in the shade, away from the elements.
Dirty air filter
A clogged or dirty air filter prevents air from getting to your engine, leading to poor combustion and overheating.
Too little air results in a “rich” gas mixture that could pool outside the engine and ignite, resulting in backfire.
SOLUTION: As with other dirty components, you’ll need to clean the air filter! Foam and hybrid filters can be washed, but paper filters should be brushed dry with soft bristles to prevent tearing.
You should also replace the filter at least once every mowing season or every 300 operating hours.
Dirty spark plug
A dirty, damaged, or worn spark plug doesn’t spark properly, which means it won’t ignite the gas inside the chamber.
The gas then reaches the hot exhaust muffler and combusts – leading to backfire.
SOLUTION: Check the spark plug for any oily residue, soot, or dirt. If the parts are intact but just dirty, then you can clean the spark plug.
A greasy spark plug can be a sign of too much oil in your mower, so have a look at the reservoir and drain it if necessary.
Any signs of damage mean the spark plug needs to be replaced. It’s also best to change it every 25-30 operating hours.
Dirty or faulty carburetor
If your mower backfires when starting, it could be due to a dirty carb! This component ensures a correct mix of air and gas in the engine.
A dirty or blocked carb can fail and cause backfire. The fault usually lies in a stuck float or blocked jets.
SOLUTION: You’ll need to clean the carburetor. Usually, you won’t even need to remove it – some products, like Gumout Carb and Choke Cleaner, can be sprayed directly into the carb.
Compressed air can also help blow out any built-up dirt and debris. Never introduce moisture, though, as this could compromise the carb and engine.
It’s best practice to clean the carburetor on its first use after a long time in storage (e.g. over winter). Then clean it every few uses during mowing season.
Poor carburetor adjustment
Adjusting (or “tuning”) a carb is a way of ensuring the correct mix of gas and air in an engine. Mowers experience wear over time, so proper carburetor adjustment ensures everything works smoothly.
A carburetor adjustment that’s too “lean” means there’s too little fuel and too much air coming through. This leads to improper ignition and engine backfire.
On the other hand, carburetor adjustment that’s too “rich” means too much fuel and too little air. This can cause the gas to leak out and ignite in the exhaust.
SOLUTION: Access the carburetor and check to see if it’s poorly adjusted. Consult your manual or the manufacturer’s website for instructions.
If you’re unsure about the steps to adjust the carb, take your power tool to a small engine repair shop.
Cold starts can also lead to a rich air-fuel mix. Be sure to prime or warm up the motor to decrease the risk of backfire.
Faulty flywheel key
If your mower runs rough or backfires when starting, the flywheel key may be misaligned or damaged.
This can happen if you’ve hit the mower on an obstacle, run over debris (such as rocks), or accidentally knocked the machine over.
Any misalignment or damage to the key breaks the connection to the transmission. This interferes with the engine’s starting, causing a backfire.
SOLUTION: Locate the key and either realign it or replace it! Check your engine model and manual for instructions on how to access the flywheel.
Ignition timing or ignition delay
Any damage or blockage in the ignition system can create a delay in the timing, causing a spark before the valves have closed. This can result in backfire.
Misaligned valves, a failing coil, or even damaged filters can lead to a delay in ignition. But engine timing also becomes unregulated with time and use.
SOLUTION: This depends on the make and model of your mower. Some engines can be tuned or have the timing adjusted (e.g. Briggs and Stratton motors), but others may need professional attention.
Check your owner’s manual for more information.
Compression problems
Engine compression is the process of an engine piston generating pressure to compress air and gas within the cylinder before ignition.
Too much pressure or too quick compression results in early ignition or detonation. Meanwhile, too little pressure results in an incorrect air-gas ratio and fuel pooling along the exhaust.
Both can lead to a backfire.
SOLUTION: Inspect the fuel pump, filter, cylinder, and lines of your engine. Clogs can interfere with proper operation, causing too high or low pressure.
Excess carbon build-up, wear in piston rings, and broken valves can also interfere with compression.
You can perform a compression test at home by using a kit, such as this KAKI Petrol Engine Compression Tester Kit. However, unless you’re confident in your DIY skills, any repairs are best left to professionals.
Faulty valves
This is a less common reason for a small engine backfire, but you could have valve issues, especially for OHV (Overhead Valve) engines.
Some problems include:
- Too-small clearances
- Loose seals
- Loose valve seat
- Stuck valves
In some models, the fuel solenoid is particularly sensitive to heat. The solenoid is the valve that cuts off gas to the engine during shut-off – if it’s stuck in an open position, gas gets out and leads to a backfire.
SOLUTION: This issue requires professional attention. It can be tricky to identify a valve problem, as the symptoms can be similar to other mower issues.
These include:
- Blue or white smoke from leaking oil
- Increased oil consumption
- Compromised acceleration
- Misfiring or backfiring
A small engine repair shop can better identify a valve issue and fix it.
FAQs about Backfiring Mower
Besides learning how to fix a backfiring lawn mower, here’s some additional information that’s good to know!
Will a bad spark plug cause a lawn mower to backfire?
Yes, it can. A bad or dirty spark plug won’t ignite the fuel, which could lead to it igniting in the exhaust and causing a backfire.
Does backfire harm my mower engine?
Yes – but not always. A backfire could damage the engine and other components as the fuel ignites outside of the combustion chamber.
The occasional backfire while starting isn’t a cause for concern, but repeated backfires can indicate systemic issues. They can also cause damage over time as engine components keep getting subjected to high temperatures.
Backfire could also injure you if you’re too close when it happens.
Briggs and Stratton do indicate on their site that “backfire through the carburetor will not harm the engine.”
How to prevent mower engine backfire?
Regular maintenance helps you stay on top of your mower’s condition and spot any wear, damage, or broken parts.
Even the best lawn mowers wear out with time and use, but they’ll last longer if you clean and replace parts as needed.
You should deep clean your mower at least twice a year, but ideally after every 25 hours of use. This gives you a chance to check for damage and prolong the lifespan of your machine.
